1) Field of the Invention
The present invention relates to the manufacture of articles such as fasteners and, more particularly, relates to an apparatus and method for reducing the grain size of materials through an angular extrusion process and forming the articles therefrom.
2) Description of Related Art
Articles such as fasteners, clips, brackets and the like that are used in the aerospace industry, where weight and strength are of critical concern, typically are subjected to repeated cycles of shear, compressive, and/or tensile stresses over the life of the articles. As a result, the articles must exhibit good mechanical strength and fatigue resistance and preferably not be unduly heavy. In addition, because the articles may be exposed to the ambient environment, including moisture and temperature fluctuations, the articles must have good corrosion resistance and resistance to thermal stresses.
To address the strength and weight requirements, some articles such as rivets are typically formed of materials having high strength-to-weight ratios, such as aluminum and aluminum alloys that are hardened by cold working or precipitation hardening. Advantageously, a number of high strength aluminum alloys are available that are lightweight, and also have relatively high fatigue and corrosion resistance. A variety of heat treatments can be performed to achieve the desired properties of the materials. For example, heat treatments for rivets, including quenching, solution treating/annealing, and precipitation-hardening aging are discussed in U.S. Pat. No. 6,403,230 to Keener. Such heat treatments can be performed during or after the manufacture of the rivets. Often, multiple heat treatments are performed during manufacture to offset cold working effects that result during the formation of the rivets. For example, heat treatments such as annealing can be used to increase the formability of the material during manufacture. Following the formation of the articles, the desired mechanical properties of the articles can be achieved by other heat treatments, such as precipitation hardening or aging. Unfortunately, the various heat treatments required during such a manufacturing process are time consuming and increase the cost of the finished articles. Additionally, if the heat treatments are conducted improperly, undesirable mechanical properties can result in the articles.
Thus, there exists a need for an improved apparatus and method for manufacturing articles having favorable mechanical properties such as strength, toughness, formability, and resistance to fatigue, corrosion, and thermal stresses. Preferably, the method should reduce the amount of heat treating that is required during manufacture. Additionally, the method should be cost effective and compatible with materials that have high strength-to-weight ratios.